Periodic Trends
Atomic Radius
The atomic radius is the distance from the electron from the proton. The size of the electron cloud increases
as the principal quantum number increases. So...as you look down the periodic table, the size of atoms in each
group is going to increase. When you look across the periodic table, you see that all the atoms in each group
have the same principal quantum number. However, for each element, the positive charge on the nucleus
increases by one proton. This means that the outer electron cloud is pulled in a little tighter. One periodic
property of atoms is that they tend to decrease in size from left to right across a period of the table.
OVERALL: Atomic radii increase as you go down and left on the periodic
table.
Atomic Radius of Ions
OVERALL: Radius of cations is decreased (lose electron) and anions increased (gained electron).
Ionization Energy
Ionization energy is the energy needed to remove the most loosely held electron from an atom. Ionization
energies are periodic. The ionization energy tends to increase as atomic number increases in any horizontal row
or period. In any column or group, there is a gradual decrease in ionization energy as the atomic number
increases. Metals typically have a low ionization energy. Nonmetals typically have a high ionization energy.
OVERALL: Ionization energy increases as you go up and right on the
periodic table.
Electron Affinity
Electron affinity is the attraction of an atom for an electron. Metals have low electron affinities while
nonmetals have high electron affinities. The general trend as you go down a column is a decreasing tendency to
gain electrons. As you go across a row there is also a trend for a greater
attraction for electrons.
OVERALL: EA (electron affinity) increases as you go up and right on
the periodic table.
Electronegativity
Electron affinity is the ability of an atom to attract electrons from a bond. As the charge in the nucleus
increases, the atom is able to attract more electrons towards itself (remember that opposites attract). Since
noble gases do not bond with other elements naturally, they are not included as we consider electronegativity.
OVERALL: Electronegativity increases as you go up and right on the
periodic table.